In order to introduce the present invention and the problems that it solves, it is useful to overview a conventional CATV broadband communication system, and then examine certain prior approaches to problems encountered when attempting to introduce telephony signals into the broadband environment.
Conventional Cable Television Systems (CATV)
Cable television systems, sometimes referred to as community-antenna television (CATV) systems, are broadband communications networks of coaxial cable and optical fiber that distribute television, audio, and data signals to subscriber homes or businesses. In a typical CATV system, a single advantageously located antenna array feeding a cable network supplies each individual subscriber with a usable television signal.
Since the pioneer days, cable networks have experienced enormous growth and expansion in the United States, particularly in urban networks. It is estimated that CATV networks currently pass approximately 90% of the population in the United States, with approximately 60-65% of all households actually being connected. While cable systems originally had very simple architectures and provided a limited number of different television signals, the increase in the number of television broadcasters and television owners over the last several decades has resulted in much more complex and costly modern cable distribution systems.
A typical CATV system comprises four main elements: a headend, a trunk system, a distribution system, and subscriber drops.
The "headend" is a signal reception and processing center that collects, organizes and distributes signals. The headend receives satellite-delivered video and audio programming, over-the-air broadcast TV station signals, and network feeds delivered by terrestrial microwave and other communication systems. In addition, headends may inject local broadcasting into the package of signals sent to subscribers such as commercials and live programming created in a studio.
The headend contains signal-processing equipment that controls the output level of the signals, regulates the signal-to-noise ratio, and suppresses undesired out-of-band signals. Typical signal-processing equipment includes a heterodyne processor or a demodulator-modulator pair. The headend then modulates received signals onto separate radio frequency (RF) carriers and combines them for transmission over the cable system.
The "trunk system" is the main artery of the CATV network that carries the signals from the headend to a number of distribution points in the community. A modern trunk system typically comprises of a combination of coaxial cable and optical fibers with trunk amplifiers periodically spaced to compensate for attenuation of the signals along the line. Such modern trunk systems utilizing fiber optics and coaxial cable are often referred to as "fiber/coax" systems.
The "distribution systems" utilize a combination of optical fibers and coaxial cable to deliver signals from the trunk system into individual neighborhoods for distribution to subscribers. In order to compensate for various losses and distortions inherent in the transmission of signals along the cable network, line-extender amplifiers are placed at certain intervals along the length of the cable. Each amplifier is given just enough gain to overcome the attenuation loss of the section of the cable that precedes it. A distribution network is also called the "feeder".
There is a strong desire in the CATV and telecommunications industry to push optical fiber as deeply as possible into communities, since optical fiber communications can carry more signals than conventional networks. Due to technological and economic limitations, it has not yet proved feasible to provide fiber to the subscriber's home. Present day "fiber deep" CATV distribution systems including optical fibers and coaxial cable are often called "fiber-To-the-Serving-Area" or "FFSA" systems.
"Subscriber drops" are taps in the distribution system that feed individual 75 .OMEGA. coaxial cable lines into subscribers' television sets or subscriber terminals, often referred to as "subscriber premises equipment" or "customer premises equipment" ("CPE"). Since the tap is the final service point immediately prior to the subscriber premises, channel authorization circuitry is often placed in the tap to control access to scrambled or premium programming.
Cable distribution systems were originally designed to distribute television and radio signals in the "downstream" direction only (i.e., from a central headend location to multiple subscriber locations, also referred to as the "forward" path). Therefore, the component equipment of many older cable systems, which includes amplifiers and compensation networks, is typically adapted to deliver signals in the forward direction only. For downstream transmissions, typical CATV systems provide a series of video channels, each 6 MHz in bandwidth, which are frequency division multiplexed across the forward band, in the 50 MHz to 550 MHz region of the frequency spectrum. As fiber is moved more deeply into the serving areas in fiber/coax and FTSA configurations, the bandwidth of the coax portion is expected to increase to over 1 GHz.
The advent of pay-per-view services and other interactive television applications has fueled the development of bidirectional or "two-way" cable systems that also provide for the transmission of signals from the subscriber locations back to the headend. This is often referred to as the "upstream" direction or the "reverse" path. This technology has allowed cable operators to provide many new interactive subscriber services on the network, such as impulse-pay-per-view (IPPV). In many CATV systems, the band of signals from 5 MHz to 30 MHz is used for reverse path signals.
However, the topology of a typical CATV system, which looks like a "tree and branch" with the headend at the base and branching outwardly to the subscriber's, creates technical difficulties in transmitting signals in the upstream direction back to the headend. In the traditional tree and branch cable network, a common set of downstream signals are distributed to every subscriber home in the network. Upstream signals flowing from a single subscriber toward the headend pass by all the other upstream subscriber homes on the segment of distribution cable that serves the neighborhood.
The standard tree and branch topology has not proven to be well suited for sending signals from each subscriber location back to the headend, as is required for bidirectional communication services. Tree and branch cable distribution systems are the most efficient in terms of cable and distribution usage when signals have to be distributed in only the downstream direction. A cable distribution system is generally a very noisy environment, especially in the reverse path. Interfering signals may originate from a number of common sources, such as airplanes passing overhead or from Citizens Band (CB) radios that operate at a common frequency of 27 MHz, which is within the typical reverse channel bandwidth of CATV networks. Since the reverse direction of a tree and branch configuration appears as an inverted tree, noise is propagated from multiple distribution points to a single point, the headend. Therefore, all of the individual noise contributions collectively add together to produce a very noisy environment and a communications problem at the headend.
Present day FTSA systems facilitate the communication of signals in the reverse direction by dividing the subscriber base of a cable network into manageable serving areas of approximately 400-2500 subscribers. This allows for the reuse of limited reverse band frequency ranges for smaller groups of subscribers. The headend serves as the central hub of a star configuration to which each serving area is coupled by an optical communications path ending in a fiber node. The fiber node is connected to the serving area subscribers over a coaxial cable distribution sub-network of feeders and drops in each serving area. In the FTSA configuration, some of the signals in the forward direction (e.g., television program signals) are identical for each serving area so that the same subscriber service is provided to all subscribers. In the reverse direction, the configuration provides an independent spectrum of frequencies confined to the particular serving area. The FTSA architecture thus provides the advantage of multiplying the bandwidth of the reverse portions of the frequency spectrum times the number of serving areas.
The Desire for Telephony Service
The ever-expanding deployment of fiber optic technology in CATV systems across the country has cable operators looking to provide a whole new range of interactive services on the cable network. One area that is of particular interest is telephony service. Because of recent advances in technology as well as the loosening of regulations, the once distinct lines between the cable television network and the telephone network have blurred considerably. Currently there is a great demand for a broadband communication system that can efficiently provide telephone service over the existing cable distribution network.
Moreover, there is substantial interest expressed by telephone system operating companies in the idea of increased bandwidth for provision of new services to telephone subscribers, such as television; interactive computing, shopping, and entertainment; videoconferencing, etc. Present day "copper" based telephony service (so called because of the use of copper wires for telephone lines) is very bandwidth limited--about 3 kHz--and cannot provide for such enhanced services by the telephone companies without massive changes to the telephone networks infrastructure.
Existing communications systems, however, have not proven to be well suited for the transmission of telephony signals on the cable network. A system for transmitting telephony signals must be configured to allow single point to single point distribution (i.e., from a single subscriber to a single subscriber). However, unlike the telephone companies with their well-established national two-way networks, the cable industry is fragmented into thousands of individual systems that are generally incapable of communicating with one another. The cable network is instead ideally configured for single point to multiple point signal transmission (i.e., from a single headend downstream to multiple subscriber locations).
Moreover, CATV systems do not have the switching capabilities necessary to provide point to point communications. A communications system for the transmission of telephone signals must therefore be compatible with the public switched telephone networks ("PSTN") operated by the telephone operating companies. To be useful in the carriage of telephony signals, a CATV network must be able to seamlessly interface to a telephony network at a point where it is commercially viable to carry telephony signals. It must also provide signals that can pass to other parts of the interconnected telephone systems without extensive modulation or protocol changes to thereby become part of the international telephone system.
Telephony on Data Communications Network
One approach taken to provide a bidirectional broadband communications system is shown in U.S. Pat. No. 5,084,903 of McNamara et al., assigned to First Pacific Networks (hereinafter referred to as "FPN"). This patent describes an approach to the communication of telephony signals that appears primarily designed to operate in an office-type data communications network environment (e.g., Ethernet). Data communications networks are typically bandwidth symmetrical, that is, the forward and reverse signal paths consume equal amounts of bandwidth, and the topology is star or serial, not tree and branch. In contrast, CATV networks are bandwidth asymmetrical, with heavy allocation of bandwidth for use in the downstream direction and limited upstream bandwidth. As the present inventors have discovered, the noise problem in the upstream direction is difficult in a broadband bandwidth-asymmetrical, tree and branch topology, as contrasted with a symmetrical office-type data communications network.
The system described in the FPN patent employs two different modulation schemes for communicating information between a central headend and a plurality of subscriber nodes. For downstream communications, the FPN system transmits signals continuously in a plurality of 6 MHz bandwidth channels. In a preferred embodiment, an AM-PSK modulator is used in the downstream path. For upstream communications, the FPN system transmits packets of information in bursts to a headend using an offset quadrature phase shift keyed (OQPSK) modulator.
While the FPN communications system may be suitable for communicating telephony signals on a data communications network such as Ethernet, it does not solve certain problems that occur in the carriage of telephony signals on a broadband cable network. Due to the single point to multiple point configuration (tree and branch) of the CATV network, upstream transmissions of telephony signals have to contend with multiple noise sources as the branch signals from each subscriber are merged together toward the headend. It is believed, however, that the burst mode approach used in the reverse path of the FPN system is particularly susceptible to these noise issues. Specifically, it is believed that the framing bits and sequencing of the data streams are susceptible to interruption when an interference signal is sustained for any significant length of time (i.e., for longer than the length of a data frame) anywhere within one of the 6 MHz bandwidth channels used to carry telephony signals.
It is further believed that the interruption of the framing bits may result in the loss of content in all telephone conversations represented within the data frame interrupted. In a data communications environment, this signal interruption may only be noticeable as a slowdown on the network, and, though inconvenient, may be considered acceptable. However, such degradation of signal quality in a cable and telephony environment is undesirable and may be unacceptable.
There is no discussion in the FPN patent of any means for insertion or removal of telephony signals from and to the public switched telephone network (PSTN). The FPN system appears to provide only a local area telephone network designed primarily for inter-office communications (such as office to office intercom), as only limited access to the PSTN is suggested. There are a number of different locations in the FPN equipment where telephony signal insertion and removal could occur, but the patent does not describe any means for signal insertion or removal, or discuss any of the issues associated with signal insertion and removal. At best, it appears that telephony signals would be inserted and removed at nodes directly connected to the broadband media (e.g., the coaxial cable), as suggested at col. 3, line 30. The patent does not indicate how such insertion and removal directly from the broadband medium should best be effected, and is silent on issues involving multiple telephony channels.
Therefore, there is a need for a broadband communications system that is compatible with the existing public switched telephone networks and that is not sensitive to noise or other interference issues, particularly in the reverse path. There is also a need for a broadband communications system that is bandwidth efficient and provides a higher spectral efficiency than present systems, thereby increasing the number of subscribers that may be served by each broadband network with telephony and enhanced services offered by CATV system operators, telephone company operating companies, and others.